The present invention relates to a process for the preparation of branched 1,3-glycols and their monoesters by means of a catalytic condensation reaction of aldehydes comprising a hydrogen atom in the alpha position.
The summary reaction can be described using the following formulae: ##STR5## wherein R.sub.1 and R.sub.2 are the same or different groups, each being lower alkyl groups having 1 to 4 carbon atoms, preferably methyl groups. It is also probable that some amounts of 1,3-glycols are formed directly by means of an aldolization of the starting aldehyde by the following reaction according to Canizarro: ##STR6##
The above-described reactions have been known for about 50 years, and are described, for example, in German Patent No. DE-C-646,482. However, different technical problems in large scale production have raised and required improvements. Thus, improvements related to the preparation process are still of importance today. Another factor which has stimulated interest in the problem is the growing field of use both of the glycol as such, and of its monoester. The industrially largest raw material among the aldehydes which have a hydrogen atom in the alpha position is the isobutyric aldehyde. It is formed in considerable quantities by the hydroformylation of propene, the so-called OXO-process, as a by-product to n-butyric aldehyde.
As condensation catalysts, alkaline compounds have hitherto exclusively been used. Such compounds are most often readily dissolved in water, such as sodium hydroxide, and potassium hydroxide. Hydroxides of metals belonging to group 2A of the periodic table also show a satisfying catalytic activity. As the water is only slightly soluble in the reaction medium and inorganic hydroxides are practically insoluble in the organic phase, the condensation reaction continues in a two-phase system, and, when using the hardly soluble hydroxides of the group 2A, in a three-phase system. Most recent patents describe different methods regarding optimization of the difficultly controlled condensation reaction, but each have different drawbacks. For example:
U.S. Pat. No. 3,718,689 sets forth demands for both the purity of the isobutyric aldehyde--less than 0.5% of water and less than 0.5% organic acids--and the necessity of powerful stirring in order to obtain a stable emulsion in a two-phase system of concentrated hydroxide-aldehyde. The reaction time is considerably long and a satisfying yield is obtained first after about 2 hours. Tables show that the reproducibility of the process is very low. Thus in accordance with Table II of the subject patent, experiments 4, 5, and 6 have been carried out at apparently identical conditions, but quite different yields of monoester were obtained (i.e. 61%, 45%, and 20%, respectively).
A similar description of an alkali catalyzed process is found in U.S. Pat. No. 3,291,821, whereby powerful stirring and intense circulation are said to be necessary. Although powerful stirring is present, the yield of the batchwise process is still low, (i.e. only about 13%).
According to German Patent No. DE-A1-3,024,496, a stream of aldehyde and a stream of NaOH solution are introduced in parallel in a batchwise reactor. In this way one controls the intense evolution of heat and avoids the very rapid start of the reaction. This is particularly important if the reactor is initially completely filled by the reactants. One serious drawback of this process is that it is impossible to use it as a continuous process. This drawback is pointed out in the specification of the subject patent.
In U.S. Pat. No. 3,703,541 the use of phenolic salts as reaction catalysts is described and, according to U.S. Pat. No. 3,091,632, sodium alkanoates are used. Both these types of catalysts increase to some extent yield and selectivity, but they have difficult requirements as to the purity of the raw material and particular requirements as to the absence of water in the reaction environment. Thus, the processes are unattractive from an industrial viewpoint.
German Patent DE-2,820,518 discloses a catalytic system consisting of alkali earth metal hydroxides and carboxylic acids and their salts, respectively. A serious drawback of this process is that one has to precipitate the alkali earth metal catalyst using gaseous carbon dioxide after the final reaction. A troublesome separation of the fine grained precipitate will then become necessary.
German Patent DE-3,403,696 discloses a two step process consisting of a condensation reaction followed by a hydrogenation of raw reaction mixture at 120.degree. C. and a pressure of 100 bars. It is not probable that such a process can be competitive against other methods described above, which only use conventional reactors provided with stirrers and tube reactors, respectively.
According to German Patent DE-A1-3,447,029 one obtains a good contact between water and an organic phase by using a tube reactor containing filler bodies --something that is well known to those skilled in the art.
U.S. Pat. No. 4,225,726 discloses a condensation reaction where tin and tin oxide, respectively, are used as catalysts. The advantage is that these catalysts make it possible to carry out synthesis reactions where aldehydes containing both one and two hydrogen atoms in the alpha position are used as a raw material. The yield of the condensation product containing only one hydrogen atom in the alpha position, as in the present invention is, when using isobutyric aldehyde, only 20%.